Mr Whiddington, The Shadow Electroscope 



113 



From these readings the mean value of VJV2 = 1*045, whence 

 C^ + Ce = 72-2 cm., and since C, = 7-2, C'^ = 65-0 cm. 



Experiment 3. To determine the Specific Inductive Capacity of 

 Ebonite. 



This can be readily carried out by using a second condenser 

 exactly similar to the one used in the previous experiment but 

 with circular plates of ebonite separating the plates instead of air. 



Then if this condenser (capacity C'j,) is placed on the electro- 

 scope in the manner of the previous experiment, and charged to 

 a potential (F^), and the sphere is used in the manner previously 

 described, the resulting collapse of the leaf will be so small as to 

 be hardly readable owing to the large capacity of the ebonite 

 condenser. It is therefore more convenient to use the air con- 

 denser of measured capacity Ca in place of the sphere. It is suffi- 

 cient to hold Ca by its outer case for earthing purposes, touching 

 its central plate momentarily to the corresponding plate of the 

 condenser Cj, on the electroscope. The potential resulting from this 

 sharing of charge (Fg) is noted. 



We then have that 



Ca+C, + Ce _ Fi 



whence 



C, + C, 



Fo 



a 



Fi/F,-1' 



in which both C^ and Cg have been previously determined by 



VOL. XX. PART I. 



